The present invention relates to a highly active and selective catalyst for the selective hydrogenation of alkynes and dienes. The present invention also relates to the preparation process and application of the catalyst.
It is well known that the ethylene-rich fraction contains 0.3-3% of acetylene impurity, which is a poison to the polyethylene catalyst and affects the proper proceeding of the polymerization reaction of ethylene. In order to prevent the aforesaid poisoning from taking place, catalytic selective hydrogenation is generally used to convert the acetylene in the ethylene-rich fraction to ethylene. But in the reaction of selective hydrogenation to remove acetylene, the acetylene adsorbed on the bearer surface can easily be converted into unsaturated C4 hydrocarbons, such as 1,3-butadiene, etc. through oligamerization. These unsaturated C4 hydrocarbons would further react with acetylene or ethylene or other unsaturated hydrocarbons to form C6-C24 oligomers, which are commonly referred as xe2x80x9cgreen oilxe2x80x9d and inevitably adhere onto the catalyst for alkyne selective hydrogenation. The adherence of the xe2x80x9cgreen oilxe2x80x9d onto the catalyst for alkyne selective hydrogenation would lead to the gradual drop of the activity and selectivity of the catalyst for selective hydrogenation and the shortening of the operation period. This would result in a frequent regeneration and shortened lifetime of the catalyst, and thereby an increase in the operation cost.
The catalyst for the selective hydrogenation of alkynes and dienes is obtained by loading a noble metal, such as palladium, on a porous inorganic material, such as disclosed in U.S. Pat. No. 4,762,956. In order to enhance the selectivity of the catalyst and alleviate the deactivation of the catalyst due to green oil formed by the oligomerization reaction during hydrogenation, a promoter such as Group IB element (e.g., Pdxe2x80x94Au as disclosed in U.S. Pat. No. 4,490,481; Pdxe2x80x94Ag as disclosed in U.S. Pat. No. 4,404,124; and Pdxe2x80x94Cu as disclosed in U.S. Pat. No. 3,912,789), or alkali or alkali earth metal (such as disclosed in U.S. Pat. No. 5,488,024) promoter is added. The support is alumina, silicon dioxide (such as disclosed in U.S. Pat. No. 5,856,262), or honeycomb cordierite (such as disclosed in CN 1176291), etc.
The traditional hydrogenation catalyst generally has a larger specific surface area to provide the catalyst with a sufficient activity. The depth of hydrogenation is generally controlled through controlling the amount of the introduced hydrogen and the selectivity of acetylene hydrogenation into ethylene is regulated through the addition of trace amount of CO. A concentration of CO over a certain level would cause the poisoning of the catalyst. An excessive amount of hydrogen would cause not only the hydrogenation of mono-olefins, but also too steep elevation and even xe2x80x9crun awayxe2x80x9d of the temperature of the catalyst bed.
Therefore, the supported palladium catalyst of the prior art is not suitable for the hydrogenation of the fraction containing larger amounts of hydrogen and CO. Especially in the xe2x80x9cfront-end hydrogenation to remove acetylenexe2x80x9d technology (that is, to remove acetylene first, then to separate methane and hydrogen), because of the high concentration of hydrogen (about 10-20 mol %) and CO (500-5000 ppm (mol)) in the reactant stream, the requirement for the activity and selectivity of the hydrogenation catalyst is even higher. In fact, the key to the improvement of the separation flow of the front-end hydrogenation acetylene is to increase the selectivity and activity of the selective hydrogenation catalyst and lower the sensitivity of the catalyst to the concentrations of hydrogen and CO.
In order to alleviate the effect of the variation in CO concentration on the activity and selectivity of the catalyst, U.S. Pat. No. 4,404,124 proposes a palladium catalyst with a very thin shell, into which silver is added as a promoter component. The patent points out that the palladium component should be distributed within 300 xcexcm of the exterior layer of the catalyst particle and the silver component should be uniformly distributed in the catalyst. CN 95107324.9 discloses a selective hydrogenation catalyst for hydrogenating in C2 or C3 alkyne into the corresponding alkene, wherein at least 80% contents of Pd and IB group metal distribute within the shell constituted by the thickness of r to at least 0.8 r of the support of the alumina balls or the extrusions. It can be seen from the examples of the invention that the stability of the catalyst is elongated from 22-66 h for the catalysts in the comparative examples to 85-121 h for the catalysts in the examples according to the present invention. Even so, the catalyst still needs to regenerate every 3-4 days. Besides, the comparative reference does not mention that the catalyst can be used at high hydrogen concentrations and the hydrogenation feed stock does not contain CO, which is a poison to palladium catalysts. The comparative reference does not disclose a catalyst for selective hydrogenation which can be used at high concentrations of hydrogen and CO and has a operation lifetime over hundreds hours.
It is surprisingly discovered by the present inventors that it is possible to decrease the yield of the green oil and elongate the lifetime of the catalyst to over about 1000 h by simultaneously distributing palladium and a Group IB metal in the catalyst body in a depth more than about 300 xcexcm from the support surface. The selectivity for alkyne and diene hydrogenation is higher than about 90%. With a space velocity of about 12000-15000 hxe2x88x921, acetylene can be removed to less than about 1 ppm, thus the activity and selectivity of the catalyst is greatly increased. The applicable range of the catalyst is wide and its catalytic performance is not affected by the variation in the concentration of CO. The content of hydrogen in the reactant stream can be about 1-30 mol % and the content of CO can be about 0-5000 ppm. The catalyst is suitable for both fore-hydrogenation and post-hydrogenation.
The present invention provides a catalyst which has low yield of xe2x80x9cgreen oilxe2x80x9d, long service lifetime, and high activity and selectivity and is applicable to the selective hydrogenation of alkynes and dienes with various concentrations of hydrogen and CO.
The present invention also provides a process for preparing the catalyst of the present invention.
The present invention provides the application of the catalyst of the present invention to the selective hydrogenation of mixed components containing alkynes and dienes to convert them into the corresponding monoolefins.
Another way to raise the selectivity and activity of the catalyst for selective hydrogenation is to improve the pore structure of the support. However, since the conventional catalyst support has lots of micropores ( less than 20 nm) and minipores (20-50 nm), there the catalyst prepared by loading an active component such as palladium on the support surface is used to hydrogenate alkynes and dienes, a lot of green oil is produced and the selectivity is low. To solve the above problems of the prior art, the support with macropores and low surface acidity is prepared generally by raising the calcination temperature, such as disclosed in U.S. Pat. No. 4,762,956. But the catalyst prepared with such a support has a low dispersion of palladium and therefore a low activity for hydrogenation.
It is surprisingly discovered by the present inventors that a macroporous support with low surface acidity can be prepared by calcining the chemical composite support of alumina and titania at lower temperatures. The support endows the catalyst with higher activity and selectivity, low yield of the green oil, and resistance to poisoning by arsenic sulfide. The operation period increases by about 0.5-1.0 time and the lifetime increases by over one time compared to the prior catalyst.
The present invention provides a support with which catalyst of high activity and selectivity for selective hydrogenation can be made. This support is particularly suitable for preparing the alkyne front-end hydrogenation catalyst.
The present invention also provides a process for preparing a catalyst support of the present invention.
The present invention provides a selective hydrogenation catalyst, which comprises:
an inorganic oxide support selected from alumina, titania, and a chemical composite of alumina and titania;
a major active component Pd of a content of about 0.002-1.0% based on the total weight of the catalyst; and
one or more promoters selected from Group IB metals and the mole ratio of palladium to the Group IB metal is in range between about 1 and about 20;
wherein the major component palladium and the promoter are uniformly dispersed together in the catalyst body within a thickness between the support surface and the depth of more than about 300 xcexcm.
The present invention provides a process for preparing the selective hydrogenation catalyst, which comprises the following steps:
preparing an inorganic oxide support, which is selected from alumina, titania, and a chemical composite of alumina and titania;
impregnating the active component, wherein the inorganic oxide support is impregnated with a mixed solution of water soluble inorganic salts of palladium and a Group IB metal to allow the active component palladium and the group IB metal promoter to be absorbed into the pores of the support together and uniformly distributed in the catalyst body within a thickness between the support surface and the depth of more than 300 xcexcm; the inorganic salts of palladium and the Group IB metal is dissolved in deionized water to form an impregnation solution, the total amount of which equals the pore volume of the support; the pH value of the impregnation solution is adjusted to be about 1-4 to control the dispersion depth of the active component and the promoter and then the impregnated support is dried at about 80-150xc2x0 C.; and
decomposing the catalyst, wherein the impregnated support is decomposed in an air atmosphere at about 300-800xc2x0 C. for about 2-10 h. The active component and the promoter of the catalyst exist in the form of oxides in the catalyst body within a thickness between the support surface and the depth of more than about 300 xcexcm.
The present invention provides a process for preparing the chemical composite of alumina and titania. The process comprises the following steps: MAlO2 and soluble titanium salt are dissolved in water at about 20-80xc2x0 C., and then the mixture is neutralized with an MOH solution to form a co-precipitate of aluminum-titanium hydroxide, which is stirred for about 10-30 h to form uniform crystal particles. The resultant is filtered and washed away the M+ and acid radical negative ions with deionized water and then the derived aluminium-titanium hydroxide is dried at about 100-150xc2x0 C. After pulverizing, the solid sample is formed by kneading and then calcined at about 800-1100xc2x0 C. to obtain the chemical composite support of alumina-titania, wherein the M is Na, K or Li.